Electrophotographic photoreceptors generally consist of an electrically conductive base and a photoconductive photosensitive layer, which is required to have (1) great charge-retaining ability, (2) great image-forming ability and (3) high stability. The charge-retaining ability is the ability of the photosensitive layer to retain positive or negative ions produced by corona discharge in the charging step of electrophotographic processing, and the photosensitive layer must have great electrical resistance to exhibit great charge-retaining ability. The image-forming ability is the ability of the photoconductive material in the photosensitive layer to form an electrostatic latent image corresponding to the pattern of light illumination given to erase the charges produced in the charging step, and the photosensitive layer must have good spectral sensitivity characteristics to exhibit great image-forming ability. The term "stability" means both the electrical stability of the photosensitive layer against repeated use of the photoreceptor and the environmental stability against heat, light, etc.
Amorphous selenium is commonly employed as a photoconductive material in the photosensitive layer of conventional electrophotographic photoreceptors, because selenium has high electrical resistance and stability. Another reason is that the defects of pure selenium, i.e. difficulty in achieveing good spectral sensitivity characteristics and environmental stability (amorphous selenium is easily crystallized with heat), can be eliminated to some extent by adding suitable impurities. It is known that good spectral sensitivity characteristics can be obtained by adding tellurium or arsenic, and electrophotographic photoreceptors having a photosensitive layer made of such photoconductive composition are being used in industry. But a photosensitive layer made of tellurium-doped selenium has a primarily chained amorphous structure, and at fairly low temperatures (ca. 50.degree. C.) it is crystallized to lose its charge-retaining ability. On the other hand, a photosensitive layer made of arsenicdoped selenium has a three-coordinate amorphous structure, so it can have a fairly high thermal stability, but since it contains arsenic, it involves a potentially great hazard in its production or subsequent handling. Another impurity that is added to selenium to increase its spectral sensitivity characteristics is antimony as described in U.S. Pat. No. 3,490,903. But commerically available photoreceptors having photosensitive layers of this type contain only a very small amount of antimony, and no commercial photoreceptor that has a photosensitive layer containing a fairly high concentration of antimony has yet been made.
Photosensitive layers made of vapor-deposited films of selenium-antimony alloy cannot have the desired electrophotographic characteristics (e.g. sensitivity, dark resistance and residual potential characteristics) or great resistance to heat and wear unless the concentration of antimony is satisfactorily high, say, 5 wt% or more. The deposited film of selenium-antimony alloy is preferably formed by using the alloy as a starting material, as in the case of depositing selenium-tellurium or selenium-arsenic alloy film. But except for an alloy having a stable structure (e.g. Sb.sub.2 Se.sub.3) or a structure close to it (with an antimony concentration of about 50 wt%), vapor deposition using a selenium-antimony alloy has one great problem in that selenium is always evaporated preferentially at low temperatures to cause a change in the antimony concentration. For instance, to form a vapor-deposited photosensitive layer of an alloy having an antimony concentration of 5 to 21 wt% that is desired as electrophotographic characteristics, the alloyed starting material must be heated to 550.degree. C. or higher, but as the alloy is heated, only selenium starts to be evaporated at about 300.degree. C., and the antimony concentration fluctuates at various states of the vapor deposition, with the result that no desired film is obtained.
This problem can be solved by evaporating selenium and antimony from separate sources so as to form a vapor-deposited film of selenium-antimony alloy on the same substrate. This method enables the formation of a selenium-antimony photoconductor with high antimony concentration by vapor-depositing the alloy film on the surface of a stationary conductive plate with a small area. But a commerical electrophotographic photoreceptor must be in a sheet form of large area or it must be of drum type. If a photosensitive layer having a fairly high antimony concentration is formed on a large sheet with a conductive layer or on a conductive drum by evaporating selenium and antimony from separate sources, an electrophotographic photoreceptor exhibiting good sensitivity and charging characteristics over an extended period cannot be obtained. This defect is particularly serious in a drum type photoreceptor which experiences so great a fluctuation in antimony concentration that it has low sensitivity while causing a great buildup of residual potential. A photoreceptor having a selenium-antimony photosensitive layer containing antimony at a fairly high concentration, say, 5 to 21 wt% is described in the above-mentioned U.S. Pat. No. 3,490,903, but it has not always been possible to produce the desired photoreceptor consistently with high productivity by simply specifying the antimony concentration.